Bill Garrett, who worked as head of technology in the BBC's production arm, told the BBC Trust in May 2012 that he had "serious concerns" about how much money was being spent on DMI and the likely success of the project to create a digital content management system that removed the need for video tape editing.

DMI was eventually axed earlier in May by BBC director general Tony Hall after he said it had "wasted a huge amount of licence fee payers' money" – £98.4m. The BBC's chief technology officer, John Linwood, has also been suspended from his £287,000-a-year job.

However, almost exactly a year before Hall's decision, Garrett wrote to Patten claiming that there was also a "very significant risk" that the National Audit Office had been misled as to the progress of DMI when it investigated the issue in 2011 and took evidence from BBC executives.

After Hall closed DMI last month, Garrett wrote again to Patten to ask why the BBC Trust had "appeared to miss or even gloss over my serious concerns".

Patten replied on Tuesday, saying Garrett's warning had been heeded and – along with financial information and concerns raised by other managers – led to the BBC Trust asking for "further information from the BBC Executive in the summer of 2012".

However, one BBC source asked why the trust had moved at "such glacial speed" to close down the project.

"BBC executives had privately been voicing concerns for about two years before DMI shut, why did it take so long? And how much was spent on DMI between May of last year and work being halted on it last autumn?" the insider said.

Last summer, Patten's attention would have been focused on appointing a successor to departing director general Mark Thompson and then he and senior BBC management – including short-lived new DG George Entwistle – were engulfed by the Jimmy Savile sex abuse scandal.

However, Garrett's letter should have set alarm bells ringing at the BBC Trust. Trustee Anthony Fry told the Commons public accounts committee in 2011 he spent every meeting concerning DMI "worrying" about it.

Fry said at the time: "I attended my first finance [and compliance] committee (FCC) meeting in January 2009, and since then I do not think there has been a single meeting of the finance committee where the subject of DMI in its various guises has not been discussed.

"The longer-term loss to the taxpayer and the licence fee payer in the event that this contract goes wrong when it is managed in-house is much more serious. Am I content? No, of course I am not content. Until this is done and dusted and delivered, I am going to spend every FCC worrying the heck about this. This is a big contract."

Garrett declined to comment on Wednesday about the correspondence he sent to Patten – which was also sent to MPs and the Public Accounts Committee – but said he was bemused as to how his name had been put in the public domain after his letter was apparently leaked to the Times.

A BBC Trust spokeswoman said: ""The concerns raised in this May 2012 letter, along with other sources of information, prompted the trust to seek further information from the BBC Executive about the DMI project in summer 2012. There followed an initial review of the project, which led to spending in the majority of areas being suspended by autumn 2012, followed by the decision recently announced to bring the project permanently to a close."

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